Certain rotor flux field measurement techniques have been used in an attempt to provide motor control which is similar to that of the dc machine by controlling the stator current and frequency at the same time. However, problems still exist in practical implementation of the control algorithm. Rotor flux field orientation requires knowledge of the rotor flux position. This position can be computed by measuring or estimating the rotor flux, but it is not generally practical to modify the machine to install the flux sensors.
To solve this problem, an approach was developed to estimate the magnitude of the rotor flux using terminal voltages and currents. However, this approach had difficulties at low speed and special attention was required due to the nature of the voltage signals. One example of this is U.S. Pat. No. 4,445,080 to Curtiss. The Curtiss device makes a measurement of the amplitude or magnitude of the flux and then this is used to regulate the flux. However, this does not give direct torque control but, in fact, is an attempt to regulate the average torque of the motor. Because the Curtiss device only measures the amplitude of the flux, it does not provide for instantaneous torque control but, in effect, operates as a low performance drive control.
Thus, as can be seen, while the Curtiss device does provide for some control of torque, it does not provide for the instantaneous control of torque which is necessary for a high performance drive system. This is because the Curtiss device operates by simple flux regulation.
Another scheme to estimate the rotor flux position uses a measurement of the rotor position. The sum of a calculated slip position and the measured rotor position yields the relative rotor flux position. The required position measurement is easier to obtain and is often already available. This scheme is referred to as indirect field orientation or feedforward field orientation.
While there are certain advantages, the machine parameter dependence in the computation of slip position (velocity) affects the performance of torque response and the efficiency of the drive system. Therefore, an enormous amount of work has been done over the past twenty years to solve this machine parameter dependence problem in indirect field orientation. It was not previously known that the air gap magnetic flux (and, in particular, the peak amplitude of the fundamental component of this flux and its angular position in the air gap as the flux rotates) could be accurately measured on an instantaneous basis by using some other machine characteristic, such as the third harmonic component of the stator phase voltage. Also, it was not previously known that such air gap flux could be used to accomplish slip gain correction.
No simple and reliable technique or apparatus was previously known which, when used in combination with an operating alternating current machine, would reliably and automatically determine the flux peak amplitude and relative position using only a sensed third harmonic component of the stator phase voltage.
Such instantaneously existing information about the peak amplitude and location of a flux such as the air gap flux, would be very useful in control devices and methods for regulating alternating current motor variables. Moreover, such control devices would themselves also be new and very useful, as would be the methods associated with their operation and use.
Electric motors consume much of the electric power produced in the United States. For example, motors consume about two-thirds of the total U.S. electrical power consumption of about 1.7 trillion kilowatt-hours. Over 50 million motors are estimated to be in use in U.S. industry and commerce with over one million being greater than 5 horsepower (hp). Over 7500 classifications for induction motors exist in the size range of 5 to 500-hp.
Although the efficiency of electrical machinery is improving, the efficiency of the typical squirrel cage induction motor ranges from about 78 to 95 percent for sizes of 1 to 100-hp. Thus, substantial energy savings can still be achieved. Energy can be saved in conventional constant speed applications when load conditions change considerably. Induction motor operation at normal operating conditions can result in high efficiencies by use of a favorable balance between copper and iron losses. Iron losses dominate at light loads. Thus, energy is saved by reducing motor magnetic flux at the expense of increasing copper losses so that an overall loss minimum can be maintained. However, the cost of the controller needed to adjust the motor flux is substantial.
In contrast to constant speed motor systems, variable speed induction motor systems characteristically involve variable torque loads over a range of speeds. Typical applications include compressors, pumps, fans and blowers of the type used in air conditioners, heat pumps, and the like. In these applications, improvement in operating efficiency is possible more economically because a controller for developing the optimum flux condition is derivable from the same converter that is used to vary the speed of the drive.